1. Field of the Invention
The invention provides methods for synthesizing B18H22 as a mixture of syn and anti isomers, commonly marketed as ClusterBoron. The invention further provides isotopically enriched B18H22 prepared by the aforementioned methods. In particular, the invention relates the preparation of natural abundance B18H22, 10B-enriched B18H22 and 11B-enriched B18H22.
2. Background.
Large boron hydride compounds have become important feed stocks for boron doped P-type impurity regions in semiconductor manufacture. More particularly, high molecular weight boron hydride compounds, e.g., boron hydride compounds comprising at least a five (5) boron atom cluster, are preferred boron atom feed stocks for molecular boron implantation.
An important aspect of modern semiconductor technology is the continuous development of smaller and faster devices. This process is called scaling. Scaling is driven by continuous advances in lithographic process methods, allowing the definition of smaller and smaller features in the semiconductor substrate which contains the integrated circuits. A generally accepted scaling theory has been developed to guide chip manufacturers in the appropriate resize of all aspects of the semiconductor device design at the same time, i.e., at each technology or scaling node. The greatest impact of scaling on ion implantation processes is the scaling of junction depths, which requires increasingly shallow junctions as the device dimensions are decreased. This requirement for increasingly shallow junctions as integrated circuit technology scales translates into the following requirement: ion implantation energies must be reduced with each scaling step. The extremely shallow junctions called for by modern, sub-0.13 micron devices are termed “Ultra-Shallow Junctions” or USJs.
Methods of manufacturing boron doped P-type junctions have been hampered by difficulty in controlling the ion-implantation process using boron. The single boron atom, being light (MW=10.8), can penetrate too deeply into a silicon substrate and diffuse throughout the substrate lattice rapidly during annealing or other elevated temperature processes.
Boron clusters or cages, e.g., boranes have been investigated as a feed stock for delivering molecular boron species to a semiconductor substrate with reduced penetration. See PCT/US03/20197.
Large boron hydride compounds, that is boron compounds having between 5 and about 100 boron atoms are preferred for use in molecular ion implantation methods for delivering boron atoms to a semiconductor substrate. Typically, there may be isomers of the boron hydride compound that exist. That is, boron hydrides with the same number of boron and hydrogen atoms that possess different chemical properties, e.g. structural isomers or stereoisomers. In addition, two or more structurally related boron hydride compounds having the same number of boron atoms but different numbers of hydrogen atoms have been isolated for various sized boron clusters. For example, pentaborane(9) and pentaborane(11) have chemical formulas of B5H9 and B5H11 respectively. Such compounds are frequently classified as closo (BnHn), nido(BnHn+2), arachno (BnHn+4), hypho (BnHn+6), conjuncto (BnHn+8), and the like. Thus, different boron hydride species, including isomers and compounds containing various amounts of hydrogen, are frequently known for boron hydrides having n boron atoms. Jemmis, et al. have provided a review of various macropolyhedral boranes and known compounds having n boron atoms and various amounts of hydrogen.1,2 
Mixtures of isomers and mixtures of n-boron atom containing boron hydrides are suitable for use in the implantation methods discussed. The molecular ions generated by the ionization process of boron hydride mixtures will have uniform and narrow weight distributions.
Current synthetic technologies for the preparation of large boron hydride molecules, e.g., boron hydride molecules with more than 12 boron atoms, are often plagued by complicated synthetic processes, low isolated yields, and/or inconsistent reproducibility.
Although there are several synthetic routes reported in the literature for the preparation of B18H22 as a mixture of isomers, they are lengthy, often result in notably low yields, are unreliable and have safety issues associated with the synthesis.
It thus would be desirable to have new methods for preparation of B18H22.